Holter monitors taperecord 24 h of the ambulatory electrocardiogram for later analysis. We will develop a portable arrhythmia monitor that is about the same size and weight but performs very different functions. Because it contains an ultralowpower microcomputer it scans the ECG for up to a month. In real time, the microcomputer continually rejects normal rhythm and searches for significant arrhythmias. Finding one, it records 8 s postincident, adds this to 8 s preincident stored data, and signals an alarm. The patient transmits the data by telephone to a central computer and waits for further instructions. In a pilot study we developed a prototype system and identified significant problem areas that require work. We will develop electrode and skin preparation techniques to minimize artifacts at the source. We will develop R-wave detection circuits and computer algorithms that optimize rhythm-detection reliability. We will perform extensive testing on volunteers. We will clinically evaluate the system by comparing this system with simultaneously worn Holter monitors. The system should be valuable for (1) monitoring post-myocardial infarction patients, (2) evaluation of antiarrhythmic drugs, (3) sleep-apnea research, (4) research on the causes leading to sudden death, and (5) any study that can accumulate long-term data on ambulatory patients.